This disclosure pertains to laser media, and particularly to laser media with controlled concentration profile of active laser ions and a method of making such a controlled concentration profile.
Solid-state lasers are used in many applications and are known to provide high output power levels which can be desirable in certain applications. Some of these applications also require short duration pulses, which may fall into the nanosecond range. In general, solid state lasers include a doped lasing medium. The lasing medium is provided inside a laser cavity and can be selected from many shapes such as cylindrical rods, rectangular slabs, discs, etc.
A laser can operate at various transverse electromagnetic (TEM) modes (e.g., TEM00, TEM01, TEM10, TEM11, TEM20, TEM02, TEM12, TEM21, TEM22, etc.), depending upon the geometry of the cavity of the laser and the gain medium, i.e., the lasing medium. When a laser operates in several transverse modes, i.e., multimode, the total intensity profile of the laser beam is a superposition of the intensity profiles of all existing transverse modes. Multimode operation can provide the maximum overall intensity. Multimode operation may be desirable if high output power is wanted. Multimode operation can, however, affect the spatial coherence of the laser and can cause “hot spots” within the lasing medium where the modes interfere to create energy overlap. These “hot spots” can lead to optical damage or they may change the mode supported by the laser cavity to an unstable output. Therefore, in some circumstances, it may be desirable to operate in a limited number of transverse modes or in a single transverse mode such as in the fundamental TEM00 mode. In addition, operating in a low order TEM modes can produce an output laser beam that has limited divergence while providing a relatively high brightness.
One conventional technique that has been used to limit the number of transverse modes or discriminate against higher-order modes is to provide an aperture inside the laser cavity. The aperture has the effect of blocking higher order modes having a larger spatial extent. In effect, only the Gaussian peak of the TEM00 fundamental transverse mode is allowed to pass through the aperture. One price to pay for a “cleaner” fundamental transverse mode TEM00 is lower power because the radiation beam inside the active laser medium interacts with less of a population inversion. Another conventional technique in reducing the number of transverse modes or discriminating against higher-order transverse modes is by varying the reflectivity of the output coupler mirror across the transverse dimension. Other structures in the lasing cavity can be used to correct beam quality as well. A Gaussian reflector can be used to reduce higher order TEM modes of the beam. However, incorporating an optical aperture or other structures in the laser cavity adds complexity, cost, and additional optical surfaces that may be susceptible to optical damage.
Instead of incorporating an optical aperture or other structures into the laser cavity, another conventional method decreases a concentration of laser active ions near the surface region of the laser medium by converting the laser active ions into laser inactive ions. The conversion of laser active ions into laser inactive ions is performed in a reducing environment at relatively high temperature by removing oxygen ions from the laser medium initially containing oxygen ions. The removal of oxygen ions results in a valence reduction of active laser ions in the same surface region which changes the laser active ions of one valence state into laser inactive ions of another valence state. The conversion of the laser active ions into laser inactive ions is driven by diffusion of oxygen vacancies. However, due to the slow diffusion rate of the oxygen vacancies, the process affects only a small portion of the laser active ions near the surface of the laser medium. Calculations show that at process temperatures near the melting point of the laser medium, a time period greater than 10 days is required to affect 50% of the laser active ions in a layer of less than 0.1 mm in thickness from the surface of the laser medium. As a result, a conversion of the active laser ions to inactive laser ions confined to near the surface of the laser medium provides only limited discrimination against higher-order transverse modes.
Therefore, there is a need in the art for a laser that can produce uniform high intensity beam quality without incorporating an optical aperture or other structures into the laser cavity or in the case of the laser medium without being limited to a conversion of the active laser ions to inactive laser ions to only near the surface of the laser medium.